Structure for reducing mismatch between symmetrical and asymmetrical transmission line and fast rise time generator utilizing same



w. M. GROVE 3,450,902

ETRICAL INE AND FAST RISE TIME GENERATOR UTILIZING SAME Filed March 21, 1966 June 17, 1969 STRUCTURE FOR REDUCING MISMATCH BETWEEN SYMM AND ASYMMETRICAL TRANSMISSION L m wLzmm w 3 OF N T I i 1 J 1 llll I l. E2; 225;; h 55 $22 was :22: h u at 588208 23;: Q H M253 so 552:: a: 91w w H E3522 2; mm wv 7 mm w IIIIIIIILF l I I I I m. ML

INVENTOR WAYNE M. GROVE ATTORNEY United States Patent C) US. Cl. 307--260 Claims ABSTRACT OF THE DISCLOSURE The line conductor of a transmission line including a line-above-ground plane section connected between coaxial input and output sections is offset and enlarged beginning in the coaxial input section and ending in the coaxial ouptut section. This reduces the spacing between the line conductor and the reference conductor along the line-above-ground plane section. A tunnel diode is connected in shunt with the transmission line in the lineabove-ground plane section. Impedance matching circuits provide a constant impedance looking from the tunnel diode towards the coaxial input sections and looking from the coaxial output section towards the tunnel diode.

This invention relates to fast rise time signal generators and to transmission line apparatus for use in providing the same.

It is an object of this invention to provide a signal generator having a reflection coefllcient when driven from its output terminals of less than one-tenth and for generating signals having a ten to ninety percent rise time of approximately twenty-five picoseconds and an overshoot and undershoot of less than five percent.

Another object of this invention is to provide away of connecting a tunnel diode in shunt with a feed-through transmission line which includes a network for inhibiting spurious reflections and symmetrical input and output sections without seriously degrading the rise time of the tunnel diode. This facilitates the provision of a signal generator for generating clean and fast rise time signals.

Still another object of this invention is to provide a transmission line including successive symmetrical and assymmetrical sections and means for minimizing the discontinuity in the transmission of an electromagnetic signal between these sections.

These objects are accomplished according to the illustrated embodiment of this invention by providing a transmision line including an inner conductor and an outer conductor having a continuous lower portion and a centrally discontinuous upper portion for forming a central lineabove-ground-plane section connected between coaxial input and output sections. 'In each of these coaxial input and output sections near the central and the continuous lower portion of the outer conductor is reduced relative to the spacing between the inner conductor and the discontinuous upper portion of the outer conductor for concentrating a transmitted electromagnetic signal between the center conductor and the continuous lower portion of the outer conductor. This reduces the discontinuity in the transmission of an electromagnetic signal between the coaxial and line-aboveground-plane sections of the transmission line. A network for inhibiting spurious reflections is included in the line-above-ground-plane section, and a tunnel diode is connected in this network in shunt with the transmission line. The coaxial input section is connected to a source of bias and trigger signal for the tunnel diode, and the coaxial output section is connected for supplying to a utilization circuit the fast rise time signal generated by the tunnel diode in response to a trigger signal from the source of bias and trigger signal.

Other and incidental objects of this invention will be apparent from a reading of this specification and an inspection of the accompanying drawing in which:

FIGURE 1 is a schematic representation of a time domain reflectometry system employing a signal generator according to this invention;

FIGURE 2 is a out way perspective view showing in detail the structure of the tunnel diode pulser of FIGURE 1; and

FIGURE 3 is a sectional view taken along the line A-A of the tunnel diode pulser of FIGURE 2.

Referring now to FIGURE 1, there is shown a signal generator '10 comprising a tunnel diode pulser -12 and a source 14 of bias and trigger signal for the tunnel diode pulser 12. As shown in detail in FIGURES 2 and 3 this tunnel diode pulser 12 comprises a transmission line 16 including an inner conductor 18 and an outer conductor 20 having a continuous lower portion 22 and a centrally discontinuous upper portion 24 for forming a central line-above-ground-plane section 26 connected between coaxial input and output sections 28 and 30,

respectively. In each of these coaxial input and output sections 28 and 30 near the central end thereof the inner conductor 18 is offset, eccentrically enlarged, and flattened on the top to reduce the spacing 32 between the continuous lower portion 22 of the outer conductor 20 and each of the enlarged inner conductor portions 34 and 36 relative to the spacing 38 between the discontinuous upper portion 24 of the outer conductor and each of the enlarged inner conductor portions 34 and 36 for concentrating a transmitted electromagnetic signal between the continuous lower portion 22 of the outer conductor and these inner conductor portions 34 and 36. This reduces the discontinuity in the transmission of the electromagnetic signal between the coaxial and lineabove-ground-plane sections of the transmission line 16. This method of reducing the discontinuity in the transmission of an electromagnetic signal between successive transmission line sections of different configuration might also be employed, for example, with a symmetrical or an asymmetrical line between ground plane section connected to a line-above-ground-plane section by altering the configuration of the transition end of the linebet'ween-ground-plane section in a manner to concentrate a transmitted electromagnetic signal between the inner conductor and the lower ground plane. In such a case'the upper ground plane need not be discontinuous adjacent to the line-above-ground-section but may simply be spaced substantially away from the electromagnetic signal path to serve principally as a shield for the lineabove-ground-plane section.

A tunnel diode 40 cannot be advantageously connected in shunt with the coaxial section 28 or 30 because when so connected a large inductance is created which seriously degrades its rise time. Thus, the tunnel diode 40 is connected in shunt with the central line-above-ground-plane section 26 which is provided for this purpose. When the tunnel diode 40 is connected in this manner a minimal inductance is created which does not seriously degrade the rise time of the tunnel diode. This tunnel diode 40 may comprise a pill-shaped element which is fixedly positioned, for example, with its anode intimately contacting a point intermediate to the enlarged portions 34 and 36 of the inner conductor 18 and with its cathode intimately contacting the flat bottom of a circular recess formed in a platform 41 provided on the continuous lower portion 22 of the outer conductor 20. The circular wall 42 of this recess further reduces the inductance created by connecting the tunnel diode 40 in shunt with the line-.above-ground-plane section 26.

The source 14 is connected for supplying a direct current bias signal and in response to an external trigger a one nanosecond trigger signal to the coaxial input section 28 of the tunnel diode pulser 12. A tunnel diode has a current-voltage characteristic which shows low and high voltage regions of positive resistance separated by an intermediate voltage region of negative resistance. Tunnel diode 40 is provided with a load line which intersects both of these voltage regions of positive resistance so that the direct current bias signal from source 14 causes the tunnel diode 40 to operate in one of its voltage regions of positive resistance and so that the one nanosecond trigger signal from source 14 triggers it through its voltage region of negative resistance thereby causing the tunnel diode 40 to apply a fast rise time step signal to a utilization circuit 43 connected to the coaxial output section 30 of the tunnel diode pulser 12. This utilization circuit 43 may comprise, for example, a feed-through sampling oscilloscope 44 which is connected for making time domain reflectometry measurements of a system 46 under test by comparing the fast rise time step signal from the output of the tunnel diode pulser 12 with a refiected signal from the system 46 under test.

A constant impedance network is included in the lineabove-ground-plane section 26 to provide a constant impedance equal to the characteristic impedance of the coaxial input section 28 looking from the tunnel diode 40 back towards the coaxial input section 28. This constant impedance network includes a capacitor 48 which is connected in shunt with the input side of the line-aboveground-plane section 26 and includes the parallel combination of a resistor 50 and inductor 52 which is serially connected between the enlarged inner conductor portion 34 and the anode of the tunnel diode 40. The capacitor 48 may comprise a semicircular dielectric element which is fixedly positioned in contacting relationship between the enlarged portion 34 of the inner conductor 18 and the continuous lower portion 22 of the outer conductor The resistor 50 may comprise a hollow cylindrical film resistive element supported by flat circular and conductive end terminals 54 and 56. One of these end terminals 54 is fixedly positioned with its flat end in intimate contact with the flat semicircular end of the enlarged inner conductor portion 34, and the other end terminal 56 is fixedly positioned with its bottom side resting on the anode of the pill-shaped tunnel diode element 40. The inductor 52 may comprise a length of wire one end of which is attached to the resistor end terminal 54 and the other end of which is attached to the resistor end terminal 56. These elements of the constant impedance network are designed so that the bias and trigger signal from the source 14 passes through the constant impedance network without significant degradation thereby inhibiting trigger instability that might otherwise result. By making the ratio of the inductance of inductor 52 to the resistance of resistor 50 equal to the product of the capacitance of capacitor 48 and the characteristic impedance of the coaxial input section 28, a constant impedance is provided which equals the characteristic impedance of the coaxial input section 28 looking from the tunnel diode 40 back towards the coaxial input section 28.

A resistor 58 is serially connected between the anode of the tunnel diode 40 and the enlarged inner conductor portion 36 to provide an impedance match looking from the coaxial output section back towards the tunnel diode 40. This resistor 58 may comprise another hollow cylindrical film resistive element supported by fiat circular and conductive end terminals 60 and 62. One of these end terminals 60 is fixedly positioned with its flat end in intimate contact with the flat semicircular end of the enlarged inner conductor portion 36. and the other .4 end terminal 62 is fixedly positioned with its fiat end in intimate contact with the flat end of resistor end terminal 56 and its bottom side resting on the anode of the pillshaped tunnel diode element 40. The sum of the resistance of this resistor 58 and the resistance of the tunnel diode 40 in its triggered condition is made equal to the characteristic impedance of the coaxial output section 30, thereby matching the coaxial output section 30 of the tunnel diode pulser 12 to the input of the utilization circuit 43. The network comprising the constant impedance network and the matching resistor 58 inhibits spurious reflections and permits the tunnel diode pulser 12 to generate clean fast rise time signals which are useful in many applications such as the time domain reflectometry system shown in FIGURE 1.

I claim:

1. Signal transmitting apparatus for reducing the discontinuity in the transmission of an electromagnetic signal between symmetrical and asymmetrical transmission lines of different types, said apparatus comprising:

a line conductor including a first portion, a second portion adjoining the first portion, and a third portion adjoining the second portion; and

reference conductor means spaced from said line conductor and including first and second portions positioned adjacent to opposite sides of said first and second portions of the line conductor and extending along the full length of those portions of the line conductor for forming a first transmission line section of one type having a symmetrical portion in which the first portion of the line conductor is symmetrically spaced between said first and second portions of the reference conductor means;

said first transmission line section also having an asymmetrical portion in which said second portion of the line conductor is offset and altered in cross section from said first portion of the line conductor for reducing the spacing between said second portion of the line conductor and said first portion of the reference conductor means relative to the spacing between said second portion of the line conductor and said second portion of the reference conductor means to concentrate the electromagnetic signal between said second portion of the line conductor and said first portion of the reference conductor means;

said first portion of the reference conductor means also being positioned adjacent to said third portion of the line conductor and extending along the full length of that portion of the line conductor for forming a second and asymmetrical transmission line section of another type.

2. Signal transmitting apparatus as in claim 1 including a switching element connected in said second and asymmetrical transmission line section between said third portion of the line conductor and said first portion of the reference conductor means.

3. Signal transmitting apparatus as in claim 2 wherein said switching element is a tunnel diode.

4. Signal transmitting apparatus as in claim 2 wherein said apparatus includes an impedance matching network connected in said asymmetrical transmission line section to provide a constant impedance looking towards the symmetrical portion of said first transmission line section from said switching element.

5. Signal transmitting apparatus as in claim 4 wherein said impedance matching network comprises:

a first resistive element connected in series with said third portion of the line conductor;

an inductive element connected in shunt with said first resistive element; and

a capacitive element connected between said third portion of the line conductor and said first portion of the reference conductor means at a position intermediate said first transmission line section and one end of said first resistive element;

said switching element being connected between said third portion of the line conductor and said first portion of the reference conductor means at a position adjacent to the other end of said first resistive element.

6. Signal transmitting apparatus as in claim 5 including a second resistive element connected in said asymmetrical transmission line section in series with said first resistive element and said third portion of the line conductor to provide a constant impedance looking towards said switching element from a symmetrical portion of a third transmission line section adjoining said asymmetrical transmission line section, said switching element being connected between said first portion of the reference conductor means and a point intermediate said first and second resistive elements.

7. Signal transmitting apparatus as in claim 5 wherein:

said first transmission line section comprises a coaxial type of transmission line section in which said first and second portions of the reference conductor means comprise oppositely-facing portions of an outer conductor and said first and second portions of the line conductor comprise adjoining portions of an inner conductor positioned within said outer conductor; and

said second transmission line section comprises a line above-ground-plane type of transmission line section in which the part of said first portion of the reference conductor means that is positioned adjacent to said third portion of the line conductor comprises a ground plane conductor and said third portion of the line conductor comprises a line-above-ground-plane conductor.

8. Signal transmitting apparatus comprising:

a line conductor;

reference conductor means spaced from said line conductor for forming a pair of symmetrical transmission line end sections adjoining a central asymmetrical transmission line section in which the spacing between the line conductor and the reference conductor means is reduced relative to the spacing between the line conductor and the reference conductor means along the symmetrical transmission line end sections;

a first resistive element connected Within said asymmetrical transmission line section in series with said line conductor;

a switching element connected in shunt with said asymmetrical transmission line section between one end of said first resistive element and said reference conductor means;

an inductive element connected within said asymmetrical transmission line section in shunt with said first resistive element; and

a capacitive element connected in shunt with said asymmetrical transmission line section between the other end of said first resistive element and said reference conductor means.

9. Signal transmitting apparatus as in claim 7 wherein:

said switching element comprises a tunnel diode; and

said apparatus includes a second resistive element connected within said asymmetrical transmission line section, said second resistive element being connected to said one end of said first resistive element and in series with said first resistive element and said line conductor.

10 Signal transmitting apparatus as in claim 8 including:

trigger means connected to one of said symmetrical transmission line end sections for triggering said tunnel diode to generate a fast rise time electromagnetic signal; and

utilization means connected to the other of said symmetrical transmission line end sections for receiving the electromagnetic signal generated by said tunnel diode.

References Cited UNITED STATES PATENTS 2,267,371 12/1941 Buschbeck 33333 2,404,797 7/1946 Hansen 33333 X 2,734,170 2/1956 Engelmann et al. 33333 X 3,302,134 1/1967 Heiter et 211. 33324.1

FOREIGN PATENTS 704,050 2/ 1954 Great Britain.

40 HERMAN KARL SAALBACH, Primary Examiner.

PAUL L. GENSLER, Assistant Examiner.

US. Cl. X.R. 

